Given that I screwed up once, the likelihood that I would post untested code
is...Zero!

I tested the code.  It works.  w contains the correct result.

Tushar's function works perfectly for me. If you have headed your module
"Option Base 1" then change

to
  ReDim Rslt(x.Count )
   For I = LBound(Rslt) To UBound(Rslt)
       Rslt(I) = x.Item(I)

The function provides an extra bonus of not including any duplicate values
in the "unioned" array. However if you want to keep all original values,
incl duplicates, a different approach -

Function ArrUnion(vUnion, v)
Dim bIsArray As Boolean
Dim cnt As Long, nTop As Long
Dim i As Long, j As Long

' input value or 1xD array only (so not a 2xD range)

   bIsArray = IsArray(v)

   If bIsArray Then
       cnt = UBound(v) - LBound(v) + 1
   Else
       cnt = 1
   End If

   If IsArray(vUnion) Then
       nTop = UBound(vUnion) + 1
       ReDim Preserve vUnion(0 To cnt + nTop - 1)
   Else
       nTop = 0
       ReDim vUnion(0 To cnt - 1)
   End If

   If bIsArray Then
       For i = LBound(v) To UBound(v)
           vUnion(j + nTop) = v(i)
           j = j + 1
       Next
   Else
       vUnion(nTop) = v
   End If
''for testing only
Dim s As String
For i = LBound(vUnion) To UBound(vUnion)
s = s & i & vbTab & vUnion(i) & vbCr
Next
MsgBox s

End Function

Sub testArrUnion()
Dim A, B, C
Dim x As Long
Dim D
   A = Array("mary", "john")
   B = Array("Peter")
   C = Array("Roger", "Dick", "Harry")
   x = 123

   ArrUnion D, A
   ArrUnion D, B
   ArrUnion D, C
   ArrUnion D, x

End Sub

Regards,
Peter T

That example is beyond the scope of the code.  As the only comment in the
code indicates it works strictly on 1D arrays.

Of course, one could easily argue that the definition of union in the
example you present is not (array("a", "b"), "c",1,"b",2,3) but rather
("a","b","c",1,2,3)

In any case...

What you want to do is in fact a subset of a larger class of possible data
sources: n-dimensional arrays or variants containing arrays of variants
containing arrays of...

While a solution can be created (I would use a recursive algorithm), it is
not included in this code.  The code also doesn't explicitly handle objects,
either native or user-defined, or variables of a custom user type or....

Instead it relies on the default value, if any -- with the attendant and
potentially unintended consequences.

I'm curious, for what purpose would you want to do that.

FWIW if you try your example with the function I posted it returns exactly
what you say you would expect (but comment the testing msgbox stuff).

Regards,
Peter T